Automotive electrical systems provide a number of operating, safety and convenience functions using various control modules. These systems and control modules ultimately derive their electrical power from the vehicle battery. When the engine is not operating, the battery supplies power without simultaneously recharging thereby draining the battery making it ineffective to start the vehicle engine when required to do so. Many electronic modules have an inactive or sleep mode that reduces power consumption.
Typically, an automotive electronics module uses an input circuit employing a pull-up resistor and a voltage reference to form a voltage divider with the resistor in a multi-resistor function switch to generate a voltage representing the selected switch position. The module microcontroller periodically checks the voltage level on this circuit to determine whether any action is required. Unfortunately, this periodic checking along with the bias current provided to the switch's resistors consumes power that will drain the battery if it operates when the engine is off. Also, the amount of power consumed depends on the resistance selected by the multi-function switch.
Another method compares the voltage applied by the multi-resistor function switch with a fixed voltage reference. This method provides a wakeup signal only when the switch is placed in a certain position. Accordingly, it will be appreciated that it would be highly desirable to have a circuit to wakeup the microcontroller when the function switch is moved from any position to any other position that uses very little power.